The invention concerns electrical pickups for acoustic guitars. Acoustic guitars, which are the traditional form of guitar, produce a significant output of direct sound energy, largely due to the ability of the body of the guitar to pick up and amplify the vibrations of the strings. As a result of this mechanism, the body contributes considerably to the tonal quality of the sound produced by the guitar. Acoustic guitars produce sufficient direct sound output for them to be usable without amplification when played in small rooms in front of small audiences. To be heard in larger auditoriums, amplification is necessary. For amplification to be used, some means for picking up the sound output of the guitar must also be used.
Electrical pickups for acoustic guitars must be distinguished from electrical pickups for electric guitars, because the primary mechanism by which each kind of guitar produces sound is quite different. Electric guitars produce sound by using one or more electric coils to pick up the vibration of the strings (which must be of a magnetic material, normally steel) in a magnetic field. The electrical output of the coils is then amplified and the amplified signal is then reproduced by means of a loudspeaker. Electric guitars produce relatively little direct sound energy themselves, and are totally reliant on amplification if they are to be heard by more than only the player. Unlike the body of an acoustic guitar, the body of an electric guitar contributes little to the direct sound energy output and to the tonal quality of the sound produced by the loudspeaker.
The conventional approach to picking up the sound on an acoustic guitar is to use a microphone mounted on a stand and directed towards the top of the guitar. Using a microphone works quite well for solo or small ensemble performances of classical music, but presents at least three problems in performances of more popular music: (1) it seriously restricts the player's ability to move around during the performance; (2) it may pick up too much noise from the action of the player's fingers and hands on the strings and top of the guitar (such noise will be called "top noise"); (3) it may pick up its own amplified output, leading to acoustic feedback problems; and (4), when the player shares the stage with loud instruments such as drums, keyboards, and electric guitars and basses, it may present severe problems in achieving the desired sound balance because the acoustic guitar microphone picks up sounds from these other sources in addition to the acoustic guitar. As a result of these problems, there has for a number of years been a tendency towards using self-contained acoustic guitar pickups which allow the acoustic guitar itself to produce an electrical output signal, which is then fed by a long cable, or a radio-frequency or infra-red transmitter/receiver arrangement to suitable amplification and loudspeaker equipment. Such a self-contained pickup arrangement immediately solves problem 1, and, properly designed, can solve problems 2 and 3.
Because it is desirable not to use steel strings on acoustic guitars, and acoustic guitars therefore lack the fundamental mechanical-to-electrical transducer mechanism of the electric guitar, the considerable amount of art relating to electric guitar pickups is not applicable to acoustic guitar pickups.
Basic requirements for a self-contained acoustic guitar pickup can be stated as follows: (1) the pickup must convert the mechanical vibrations of the guitar strings and body into an electrical signal; (2) the pickup must pick up some top noise, but top noise pick up should not be excessive; (3) the pickup should pick up the sound of the guitar without adding colorations of its own; (4) the pickup (together with any amplification required) should have a high electrical signal-to-noise ratio; (5) the pickup should not pick up hum, buzz and other externally induced noise; (6) the pickup should pick up the output of each string more-or-less equally; (7) it should be easy to install the pickup in the guitar, and should require a minimum of modifications to be made to the guitar itself. Some acoustic guitars are valuable antiques, the value of which can be reduced if extensive machining operations are required to accommodate a pickup; and (8) it should be easy to remove the pickup and restore the guitar to pickup-less operation.
A number of acoustic guitar pickups are already commerically available. The FRAP pickup, described in U.S. Pat. No. 3,624,264 uses three ceramic or crystalline piezoelectric transducers orthogonally mounted on three of the walls of a small box-shaped enclosure which is filled with silicone rubber. The pickup is simply attached to the body of the guitar by means of a wax or other suitable adhesive. The transducers are arranged so that one transducer detects motion along the x axis, another detects motion along the y-axis, and the third detects motion along the z-axis. The outputs of the transducers are fed in parallel into a buffer amplifier. This pickup meets requirements (1) through (3), (6), and (7) stated above. However, it is very expensive; its electrical output is low, so it suffers from signal-to-noise ratio problems; and its ability to pick up equally from all of the strings is dependent on where it is mounted on the guitar. It is often mounted under the bridge towards the end of the bridge over which the higher pitched strings pass, so tends to pick up predominantly from the higher pitched strings. This disadvantage can be overcome by using two pickups, one mounted towards each end of the bridge. This has the further advantage of offering stereo operation but at the expense of doubling the already high cost of the pickup.
Another approach is that of Baggs, described in U.S. Pat. No. 4,314,495, which is a combination piezoelectric transducer and saddle. The saddle is a component of the bridge of an acoustic guitar; it is the part of the bridge on which the strings rest. Practical embodiments of the Baggs pickup differ somewhat from the configuration described in the patent. Practical embodiments use six series-connected ceramic or crystalline piezoelectric transducers, one for each string, encapsulated in epoxy resin in a brass U-shaped channel transducer housing. The transducer housing is an integral part of a saddle formed using a fibre/resin material such as that sold under the trademark Micarta. The channel construction of the transducer housing together with the suspension of the piezoelectric transducers in epoxy resin, is thought to reduce top noise (Requirement 2 is met).
Replacing an existing saddle with a Baggs pickup is not simple, however. First, most guitar saddles are 3/32" (2.4 mm) wide (i.e., in the direction of the strings running over the saddle): the Baggs pickup is 1/8" (3.2 mm) wide, so to install it, the saddle slot in the bridge must be routed out (widened) to accommodate the pickup. Moreover, pickup is longer than a standard saddle: a further routing operation is required to lengthen the saddle slot by 1/16" (1.6 mm) to accommodate the pickup. Thus, requirement (6) is not met. The changes to the saddle slot mean that if the pickup is removed, it must be replaced by a non-standard, wider than normal, saddle. Thus, requirement (7) is not met. Moreover, since the pickup includes a completely new saddle, the guitar must be re-intonated when the pickup is installed. Finally, the brass insert in the Baggs pickup makes it more rigid than a normal saddle in the direction transverse to the direction of the strings. This means that the top of the guitar flexes differently when tension is applied to the strings, which changes the action of the guitar. More adjustment to the shape of the saddle may therefore be necessary to restore the action to normal. This pickup is also relatively short lived: the plastic saddle wears considerably more quickly than a conventional bone saddle and, when the saddle wears out, the whole pickup must be replaced. Bone cannot be substituted for plastic because it does not have appropriate directional characteristics (see below). The plastic saddle also tends to break off the brass transducer housing. Each time the pickup is replaced, the guitar must be re-intonated, which is an inconvenience.
The Baggs pickup also has some inconvenient electrical properties. The plastic material of the saddle transmits vibrations in the direction from the string to the body more efficiently than in a transverse direction. This means that the transducer mounted under each string picks up vibrations from its own string much more efficiently than it picks up vibrations from adjacent strings. This property of the plastic material enables the transducers under the A and D strings to be connected out of phase with the transducers under the other four strings. This arrangement effectively reduces top noise, top noise being transmitted to the six transducers more or less equally, but causes phasing problems when the electrical output of the guitar is mixed with any signal that might include a component representing the acoustic output of the guitar.
The Fishman pickup, is described in U.S. Pat. No. 4,727,634, U.S. Pat. No. 4,774,867, and U.S. Pat. No. 4,944,634. This pickup uses six small (1/16" dia..times.0.02,"1.6 mm dia..times.0.5 mm) cylindrical ceramic piezoelectric transducers, one for each string. The finished pickup is narrow enough to fit in a standard 1/8" (3.2 mm) saddle slot. The pickup is mounted in the guitar simply by removing the saddle, removing about 1/16" (1.6 mm) from the height of the saddle, drilling a hole in the top at one end of the saddle slot for the pickup output lead, and placing the pickup followed by the saddle in the saddle slot. This pickup is therefore easy to install, but suffers from the general defects of pickups based on ceramic or crystalline piezoelectric transducers discussed below. Moreover, the pickup is quite complex, since it requires separate components to mount the individual transducers resiliently, to interconnect them, and to shield them from outside interference.
All acoustic guitar pickups based on ceramic or crystalline piezoelectric transducers suffer from a number of common problems: (1) such transducers have mechanical resonances in the audio frequency range that colour the sound of the guitar; (2) the mechanical mountings of such transducers have their own mechanical resonances in the audio frequency range that further colour the sound of the guitar; and (3) such transducers are small and are thus awkward to handle in such assembly operations as attaching wires to them, etc.
A new form of piezoelectric material, a polarized homopolymer of vinylidene fluoride (PVDF) has recently become available. This material is sold under the trademark "KYNAR." Full information about this material can be found in the KYNAR Piezo Film Technical Manual (Pennwalt Corporation, 1987). This piezoelectric material is a plastic film which is available in a number of thicknesses (e.g., 28, 52, 110 microns). PVDF film has a number of properties that make it advantageous for use in acoustic guitar pickups: it has a high output voltage for a given mechanical stress; it has a low mass and a low Q, which means that it responds instantly to a mechanical input, and introduces little coloration of the sound.
Electrical contacts can be made to the material itself simply by painting a suitable lead pattern with a conductive paint, or, preferably for mass-producing, silkscreening a suitable lead pattern with a conductive ink. Attaching leads to the lead pattern presents problems, however, because of the material's low softening point and low resistance to tearing. The manufacturer suggests that a low-temperature solder can be used. This enables a reliable electrical contact to be made, but does not solve the mechanical strength problems, however.
The use of PVDF film as an acoustic guitar pickup is described at page 43 of the KYNAR Technical Manual. A piece of 28 micron thick film, about 3" by 1" is metallized on both sides. It is electrically shielded on both sides by means of a metallic foil and mechanically protected by a layer of a flexible plastic laminate. Electrical contacts are made (somehow) to the metallization on each side of the film. The complete transducer is attached to the top of the guitar, close to the sound-hole, using double-sided adhesive tape, and oriented with its long axis running in the direction of the strings so that pickup of finger noise is reduced. This type of pickup tends to pick up strings that are closer to the pickup more efficiently than strings that are more distant. The pickup placement suggested in the Technical Manual would therefore tend to give a bass-heavy output. This problem could be partially solved by using two pickups, one at each end of the bridge, in a "stereo" arrangement.
A practical embodiment of this pickup solves the lead attachment problem by using sprung mechanical contacts to pick up the electrical output of the transducer. This results in a bulky arrangement, compared with the rest of the pickup, the contact device being about 1.2.times.1.2.times.0.2 inches (30.times.30.times.5 mm).
An alternative form of acoustic guitar pickup using PVDF film is described in Kynar Piezo Film News, No. 1 (Pennwalt Corp., 1987) at page 4. The sides and bottom of standard-sized saddle are partially wrapped with a piece of transducer material about 2.8.times.0.7 inches (71.times.18 mm). The long sides of the transducer material are curved to match the curvature of the top of the saddle. The material is metallized completely on the outside and metallized in six segments, one for each string, on the inside (i.e., the side closer to the saddle). The transducer is glued directly to the saddle. There is no mechanical protection or electrical shielding; the player's hand can induce an objectional buzz into the output of the pickup if it gets too close to the pickup. This pickup is also relatively short lived: the saddle material is not as durable as bone, the material normally used for making saddles, and the whole pickup must be replaced if the saddle wears out.
This basic assembly would install directly in a standard saddle slot without any modification were it not for the large plastic connector assembly on one end of the modified saddle. To accommodate the connector assembly, the saddle slot in the bridge must be routed out to about 0.22" (5.6 mm) for a length of about 0.3" (7.6 mm); the saddle slot must be lengthened by about 0.07" (1.8 mm); and a 0.22" (5.6 mm) diameter hole must be drilled in the top at the widened end of the saddle slot. This pickup is therefore inconvenient to install, and difficult to replace if no longer desired.
Practical embodiments of this pickup are sold as part of the Gibson.TM. Symbiotic Oriented Receptor System (S.O.R.S.).